Doors used in mines operate under conditions not usually encountered by typical doors. Mine doors have door leafs that tend to be heavy and dimensionally large and are thus subject to large forces due at least in part to ventilation air flow in the mine and consequent air pressure differentials on opposite sides of a door. A leaf can be as large as 10 feet wide and 20 feet high and sometimes even larger. It can weigh more than a thousand pounds when designed for pressure differentials of seven inches of water gauge and over two thousand pounds for a pressure differential of 20 inches of water gauge. Even small pressure differentials can create large forces on the leafs because of their relatively large surface areas. It is difficult to control door leaf movement because of these forces and because of the substantial inertia associated with the heavy door leafs. Thus, it is desirable for the opening and closing of mine doors to be powered by one or more actuators, such as pneumatic or hydraulic power cylinders. From a cost standpoint, pneumatic power cylinders are preferred over hydraulic power cylinders. It is also desirable to use pneumatic power rather than hydraulic power because compressed air that may already be available in relation to other mine operations can be used to power the door installation as well, thereby obviating the need to provide a separate power supply for the door installation.
Unfortunately, pneumatically-powered mine doors are vulnerable to door leaf runaway due to compressibility of the air in the pneumatic actuator. When the resistance to door movement is high, the pressure in the pneumatic actuator must build up sufficiently to overcome the resistance. If the resistance drops off while the pressure in the pneumatic actuator is still high, the door leaf can accelerate unexpectedly and swing with great speed. This is dangerous because a rapidly swinging door leaf could easily injure a person or damage machinery. At a minimum a runaway door leaf would cause undesirable wear or damage to the door installation. Furthermore, the mine environment creates conditions that favor door leaf runaway. For example, if the door leaf opens by swinging toward the high pressure side of the door, the initial resistance to opening the door will be much higher than the resistance after the door is opened a small amount and the air pressures on opposite sides of the door leaf begin to equalize. It is also possible that a door leaf will catch on part of the floor or ceiling due to the natural convergence of the floor and ceiling caused by the overburden. Similarly, rock or other debris could obstruct movement of a door leaf. As a result of these or similar obstructions, pressure could build up in the pneumatic actuator causing the door leaf to run away when the resistance drops after the leaf has overcome the obstruction.
One strategy that has been employed to partially obviate the problem with runaway door leafs is to arrange the door so the leafs open by swinging away from the high pressure side of the door. Alternatively, a bidirectional double door can be used wherein one leaf opens by swinging away from the high pressure and one leaf opens by swinging toward the high pressure. If at least one door leaf opens by swinging away from the high pressure side of the door installation, the pneumatic actuator does not need to build up as much pressure to initiate opening. Consequently, the runaway leaf problem is alleviated to some degree. However, this is not an entirely satisfactory resolution to the runaway leaf problem. The leafs are still susceptible to runaway caused by obstructions from the floor, ceiling, or debris. Moreover, the door installation does not seal well when there is a leaf that opens by swinging away from the high pressure side of the door because the force from the pressure differential tends to move the leaf toward the open position and tends to push any sealing flaps away from the surfaces against which they are intended to seal. A better seal can be obtained by having all the door leafs in a door installation open by swinging toward the high pressure side. This way the force from the pressure differential tends to tighten the seal by pressing the door leafs and sealing flaps tightly closed.
Thus, there is a need for a mine door installation powered by pneumatic cylinders that avoids the problem noted above.